This is an ongoing study of the interaction of ionizing radiation, leukemogenic viruses, target cells, the thymus microenvironment, and genetic factors in the induction and pathogenesis of thymic lymphomas in C57BL/Ka and related strains of mice. Specifically, we plan to: 1) Study the molecular characteristics of the radiation leukemia virus (RadLV) in comparison to those of related non-leukemogenic viruses of the C57Bl/Ka strain. 2) Induce, isolate, and chracterize conditional-lethal mutants of RadLv. 3) Investigate the molecular mechanisms whereby radiation activates the virus. 4) Ascertain whether RadLV is the causative agent of the lymphomas from which it is isolable. 5) Investigate the pathogenesis of the murine lymphomas. 6) Study changes in immune responsiveness of mice during the latent period of lymphoma development, and the possibility of effective immunoprevention. 7) Investigate the evolution of autonomy of lymphomas in the autochthonous host. 8) Study the influence of genetic and differentiation-specific restriction mechanisms on susceptibility to lymphoma development.